Baker, S.D.

Effect of ABCC2 (MRP2) transport function on erythromycin metabolism (Article)

2011-05-01T00:00:00Z

The macrolide antiobiotic erythromycin undergoes extensive hepatic metabolism and is commonly used as a probe for cytochrome P450 (CYP) 3A4 activity. By means of a transporter screen, erythromycin was identified as a substrate for the transporter ABCC2 (MRP2) and its murine ortholog, Abcc2. Because these proteins are highly expressed on the biliary surface of hepatocytes, we hypothesized that impaired Abcc2 function may influence the rate of hepatobiliary excretion and thereby enhance erythromycin metabolism. Using Abcc2 knockout mice, we found that Abcc2 deficiency was associated with a significant increase in erythromycin metabolism, whereas murine Cyp3a protein expression and microsomal Cyp3a activity were not affected. Next, in a cohort of 108 human subjects, we observed that homozygosity for a common reduced-function variant in ABCC2 (rs717620) was also linked to an increase in erythromycin metabolism but was not correlated with the clearance of midazolam. These results suggest that impaired ABCC2 function can alter erythromycin metabolism, independent of changes in CYP3A4 activity.

Pharmacogenetic pathway analysis of docetaxel elimination (Article)

2009-02-01T00:00:00Z

The purpose of this study was to evaluate the affinity of docetaxel for 14 transporter proteins and assess the functional significance of 17 variants in five genes involved in drug elimination. Among the transfected models investigated, OATP1B3 (SLCO1B3) was identified as the most efficient influx transporter for docetaxel. None of the observed genotypes (SLCO1B3, ABCB1, and ABCC2) was related with docetaxel clearance in 92 white patients (P > 0.17). However, the simultaneous presence of the CYP3A4*1B and CYP3A5*1A alleles was associated with a 64% increase in docetaxel clearance (P < 0.0015), independent of both sex and CYP3A activity (as determined using the erythromycin breath test). This haplotype was also associated with increased midazolam clearance in another population (P < 0.0198). An analysis of the CYP3A locus among CEPH-HapMap samples revealed that CYP3A4*1B is present exclusively among a subset of CYP3A5 expressors. Therefore, future studies should first stratify the population on the basis of CYP3A5 genotype and then compare CYP3A activity between individuals with and without the CYP3A4*1B allele.

Influence of solute carriers on the pharmacokinetics of CYP3A4 probes (Article)

2008-12-01T00:00:00Z

We hypothesized that the assessment of baseline CYP3A4 activity is influenced by probe-specific differences in hepatocellular uptake mechanisms. There was no significant correlation between the erythromycin breath test (ERMBT) parameters and midazolam clearance in 30 cancer patients (R2< 0.01), regardless of their CYP3A5 genotype status. In cellular models overexpressing 10 different solute carriers, erythromycin uptake was significantly increased by OATP1A2 (P < 0.005) and OATP1B3 (P < 0.01). Midazolam was not a substrate for any of the tested transporters. In a separate cohort of 119 patients, 6 nonsynonymous variants in the OATP1B3 gene SLCO1B3 were identified. Individuals carrying two copies of the T allele at the 334 locus had a 2.4-fold lower value for ERMBT 1/Tmax(P = 0.001), a measure reflecting more rapid hepatic uptake. These findings suggest that differential affinities for solute carriers should be considered when selecting an appropriate phenotypic probe to allow tailored dosing of pharmaceuticals that are CYP3A4 substrates.

Population pharmacokinetic model for docetaxel in patients with varying degrees of liver function: Incorporating cytochrome P450 3A activity measurements (Article)

2008-07-01T00:00:00Z

The relationship between cytochrome P4503A4 (CYP3A4) activity and docetaxel clearance in patients with varying degrees of liver function (LF) was evaluated. Docetaxel 40, 50, or 75 mg/m2was administered to 85 patients with advanced cancer; 23 of 77 evaluable patients had abnormalities in LF tests. Baseline CYP3A activity was assessed using the erythromycin breath test (ERMBT). Pharmacokinetic studies and toxicity assessments were performed during cycle 1 of therapy and population modeling was performed using NONMEM. Docetaxel unbound clearance was lower (317 vs. 470 l/h) and more variable in patients with LF abnormalities compared to patients with normal LF. Covariates evaluated accounted for 83% of variability on clearance in patients with liver dysfunction, with CYP3A4 activity accounting for 47% of variation; covariates accounted for only 23% of variability in patients with normal LF. The clinical utility of the ERMBT may lie in identifying safe docetaxel doses for patients with LF abnormalities.

Interaction of imatinib with human organic ion carriers (Article)

2008-05-15T00:00:00Z

Purpose: The activity of imatinib in leukemia has recently been linked with expression of the organic cation transporter 1 (OCT1) gene SLC22A1. Here, we characterized the contribution of solute carriers to imatinib transport in an effort to further understand mechanisms involved in the intracellular uptake and retention (IUR) of the drug. Experimental Design: IUR of [3H] imatinib was studied in Xenopus laevis oocytes and HEK293 cells expressing OATP1A2, OATP1B1, OATP1B3, OCT1-3, OCTN1-2, or OAT1-3. Gene expression was determined in nine leukemia cell lines using the Affymetrix U133 array. Results: Imatinib was not found to be a substrate for OCT1 in oocytes (P = 0.21), whereas in HEK293 cells IUR was increased by only 1.20-fold relative to control cells (P = 0.002). Further-more, in 74 cancer patients, the oral clearance of imatinib was not significantly altered in individuals carrying reduced-function variants in SLC22A1 (P = 0.99). Microarray analysis indicated that SLC22A1 was interrelated with gene expression of various transporters, including ABCB1, ABCC4, ABCG2 (negative), and OATP1A2 (positive). Imatinib was confirmed to be a substrate for the three efflux transporters (P < 0.05) as well as for OATP1A2 (P = 0.0001). Conclusions: This study suggests that SLC22A1 expression is a composite surrogate for expression of various transporters relevant to imatinib IUR. This observation provides a mechanistic explanation for previous studies that have linked SLC22A1 with the antitumor activity of imatinib. Because of its high expression in the intestine, ciliary body, gliomas, and leukemia cells, OATP1A2 may play a key role in imatinib pharmacokinetics-pharmacodynamics.

Factors Affecting Pharmacokinetic Variability of Docetaxel (Doctoral Thesis)

2004-09-23T00:00:00Z

In the early 1960s, the US National Cancer Institute started a program aimed at the discovery of new anticancer drugs by releasing protocols for widespread screening of substances and extracts from various origins for antineoplastic activity. Of the more than 100,000 compounds from 35,000 plant species tested between 1960 and 1981, paclitaxel, a complex diterpene isolated from the Pacific Yew tree (Taxus brevifolia), proved one of the most interesting and active agents. Extensive studies on the synthesis of paclitaxel analogues and the development of structure-activity relationships have been carried out over the last few decades, and led in 1986 to the development of docetaxel, a semisynthetic taxane derivative prepared from a noncytotoxic precursor isolated from the European Yew tree (Taxus baccata). Following an extensive clinical evaluation program that started in 1990, docetaxel has currently been recognized as one of the most widely active agents available, and it has been approved in most countries for the treatment of advanced breast cancer and nonsmall cell lung cancer.

Disposition of docosahexaenoic acid-paclitaxel, a novel taxane, in blood: in vitro and clinical pharmacokinetic studies (Article)

2003-01-01T00:00:00Z

PURPOSE: Docosahexaenoic acid-paclitaxel is as an inert prodrug composed of the natural fatty acid DHA covalently linked to the C2'-position of paclitaxel (M. O. Bradley et al., Clin. Cancer Res., 7: 3229-3238, 2001). Here, we examined the role of protein binding as a determinant of the pharmacokinetic behavior of DHA-paclitaxel. EXPERIMENTAL DESIGN: The blood distribution of DHA-paclitaxel was studied in vitro using equilibrium dialysis and in 23 cancer patients receiving the drug as a 2-h i.v. infusion (dose, 200-1100 mg/m(2)). RESULTS: In vitro, DHA-paclitaxel was found to bind extensively to human plasma (99.6 +/- 0.057%). The binding was concentration independent (P = 0.63), indicating a nonspecific, nonsaturable process. The fraction of unbound paclitaxel increased from 0.052 +/- 0.0018 to 0.055 +/- 0.0036 (relative increase, 6.25%; P = 0.011) with an increase in DHA-paclitaxel concentration (0-1000 microg/ml), suggesting weakly competitive drug displacement from protein-binding sites. The mean (+/- SD) area under the curve of unbound paclitaxel increased nonlinearly with dose from 0.089 +/- 0.029 microg.h/ml (at 660 mg/m(2)) to 0.624 +/- 0.216 microg.h/ml (at 1100 mg/m(2)), and was associated with the dose-limiting neutropenia in a maximum-effect model (R(2) = 0.624). A comparative analysis indicates that exposure to Cremophor EL and unbound paclitaxel after DHA-paclitaxel (at 1100 mg/m(2)) is similar to that achieved with paclitaxel on clinically relevant dose schedules. CONCLUSIONS: Extensive binding to plasma proteins may explain, in part, the unique pharmacokinetic profile of DHA-paclitaxel described previously with a small volume of distribution ( approximately 4 liters) and slow systemic clearance ( approximately 0.11 liters/h).

Irinotecan pathway genotype analysis to predict pharmacokinetics (Article)

2003-01-01T00:00:00Z

PURPOSE: The purpose was to explore the relationships between irinotecan disposition and allelic variants of genes coding for adenosine triphosphate binding cassette transporters and enzymes of putative relevance for irinotecan. EXPERIMENTAL DESIGN: Irinotecan was administered to 65 cancer patients as a 90-min infusion (dose, 200-350 mg/m(2)), and pharmacokinetic data were obtained during the first cycle. All patients were genotyped for variants in genes encoding MDR1 P-glycoprotein (ABCB1), multidrug resistance-associated proteins MRP-1 (ABCC1) and MRP-2 (canalicular multispecific organic anion transporter; ABCC2), breast cancer resistance protein (ABCG2), carboxylesterases (CES1, CES2), cytochrome p450 isozymes (CYP3A4, CYP3A5), UDP glucuronosyltransferase (UGT1A1), and a DNA-repair enzyme (XRCC1), which was included as a nonmechanistic control. RESULTS: Eighteen genetic variants were found in nine genes of putative importance for irinotecan disposition. The homozygous T allele of the ABCB1 1236C>T polymorphism was associated with significantly increased exposure to irinotecan (P = 0.038) and its active metabolite SN-38 (P = 0.031). Pharmacokinetic parameters were not related to any of the other multiple variant genotypes, possibly because of the low allele frequency. The extent of SN-38 glucuronidation was slightly impaired in homozygous variants of UGT1A1*28, although differences were not statistically significant (P = 0.22). CONCLUSIONS: It is concluded that genotyping for ABCB1 1236C>T may be one of the factors assisting with dose optimization of irinotecan chemotherapy in cancer patients. Additional investigation is required to confirm these findings in a larger population and to assess relationships between irinotecan disposition and the rare variant genotypes, especially in other ethnic groups.